Typically, whipstocks are used to create laterals from an existing bore to reach an as yet untapped formation. Whipstocks have traditionally been fairly lengthy and have incorporated a sloping surface to direct a milling assembly through a casing wall to form an opening in the casing wall known as a window. After the window is fully formed, the milling assembly is removed and the whipstock guides a drilling assembly through the window to drill the lateral. Casings have what is known in the industry as a drift diameter. The drift diameter is the largest dimension a tool can be and still fit through the inside diameter of the casing. Typically, milling assemblies that are frequently delivered with a whipstock have had external diameters at or near the drift diameter or approximately 97% of the casing inside diameter. The angle of inclination on the whipstock face has typically been less than 3.5°. This small angle creates limitations depending the location of available exit points for laterals and location and composition of adjacent formations. The slight angle on the whipstock requires an exit point from the casing and an exit trajectory of the drill bit that undesirably penetrates an adjacent formation that might produce water or sand or it could be highly unconsolidated and difficult to drill or complete.
The apparatus and method of the present invention allows for shorter radius exits from a window than had been accomplished in the past. It employs whipstock face inclinations of greater than about 3.5° and a window mill diameter of less than 95% of the casing inside diameter. This combination allows for short radius exits and avoids overstressing the milling equipment that forms the window. Those skilled in the art will better appreciate the features of the claimed invention from a review of the description of the preferred embodiment and the claims, which appear below.